1. Field of the Invention
This invention generally relates to elongated bag-like tubular filters ("filter bags") for filtering particulate material carried in a gas or liquid stream in a filter housing and, more particularly, relates to a ceramic coated filter medium and internal support for use in gas filtration.
2. Related Background Art
Filter bags are typically mounted in filter housings or in a baghouse with supporting frames or cages positioned therein to hold the filter bags in an open, hollow tubular configuration. The bags are open at an outlet end and closed at the other end. Particulate-laden gas or liquid is directed into the baghouse and through the gas-permeable filter bags. The particulate material is filtered and retained on the exterior surface of the filter bags while the filtered gas passes through the filter bag. Periodically, a reverse purge of gas or fluid is directed into the open outlet end of the filter bags for dislodging any trapped particulate material from the bags and thus, cleaning the filter bags. Hence, filter media must be able to withstand not only the normal conditions of gas filtration, but also the periodic cleaning process to which they will be subjected.
The temperatures at which gas filtration is conducted are greatly limited by the properties of the filter media employed. Synthetic filter media typically have insufficient heat resistance beyond approximately 250.degree. C. At temperatures above 250.degree. C., loss of mechanical strength and/or thermal decomposition is observed with such conventional synthetic filter media. For temperatures up to about 300.degree. C., woven or non-woven glass can be used. However, filters of glass fibers are often brittle and do not withstand the mechanical stress associated with typical baghouse operations. For temperatures above 300.degree. C., metallic filter media may be utilized. However, due to their very high price and limited corrosion resistance, the field of application for metallic filter media is very limited. Vacuum formed fiber ceramics or sintered grain ceramics are also available. However, due to the limited filter area per ceramic filter element, substantially more space is required for the total filter, thereby resulting in elevated cost.
Hence, there is great industrial demand for inexpensive, compact filtration equipment operational within a high-temperature range of about 300.degree. C. to about 500.degree. C. or even as high as 700.degree. C. or higher. Glass fibers are an attractive filter media in this temperature range due to their desirable physical and chemical properties and low cost. However, as described previously, the use of glass fiber based filter media is limited by the brittleness of the media which results in fiber breakage due to friction and bending during use.
To reduce friction between the glass fibers, the art has suggested applying various coatings, such as silicon, starch, graphite and PTFE to glass fibers. However, above 300.degree. C., these coatings degrade, thus resulting in the loss of fiber protection and therefore, rapid destruction of the filter media.
Therefore, for economical and process reasons, a need exists for a filter medium operational at temperatures above about 300.degree. C., exhibiting good chemical resistance, long lifetime, a larger filter area per element, and an attractive price.